1. Field of the Invention
The present invention relates generally to a state determining apparatus for an exhaust gas purifier which purifies exhaust gases exhausted from an internal combustion engine, and more particularly, to a state determining apparatus for an exhaust gas purifier which purifies exhaust gases by adsorbing hydrocarbons contained in the exhaust gases by an adsorbent.
2. Description of the Prior Art
One type of internal combustion engine is provided with an adsorbent disposed in its exhaust system for adsorbing hydrocarbons in exhaust gases upon starting the engine. The adsorbent has, for example, zeolite on its surface, such that hydrocarbons within exhaust gases enter into pores of the zeolite and adsorbed by the adsorbent when the hydrocarbons pass through the adsorbent. As the adsorbent is heated to a predetermined temperature or higher (for example, 100-250xc2x0 C.) by exhaust gases, the adsorbent desorbs once adsorbed hydrocarbons which are recirculated to the internal combustion engine through an EGR pipe and the like. While the adsorption and desorption of hydrocarbons are repeated in the adsorbent in the foregoing manner, a long-term use of the adsorbent may lead to a gradually increasing amount of residual hydrocarbons which could not be desorbed, and broken pores of the adsorbent. As a result, the adsorbent is deteriorated, causing gradually degraded capabilities of adsorbing hydrocarbons in the adsorbent. When the internal combustion engine is started in such a state, hydrocarbons not adsorbed by the adsorbent are emitted to the outside. Thus, a need exists for determining the state of the adsorbent, in particular, a deterioration thereof.
The applicant has proposed a deterioration determining apparatus for determining a deterioration of such an adsorbent, for example, in Japanese Patent Application No. 2000-338375. This deterioration determining apparatus takes advantage of a proportional relationship found between the capabilities of the adsorbent to adsorb hydrocarbons and moisture, and detects the humidity of exhaust gases which have passed through the adsorbent by a humidity sensor to determine degraded capabilities of the adsorbent to adsorb hydrocarbons and moisture, i.e., the deterioration of the adsorbent. More specifically, the deterioration determining apparatus sets a predetermined time required for a detected humidity of the humidity sensor to rise by a predetermined value associated with a gradual rise in humidity of exhaust gases which pass through the adsorbent, while moisture in the exhaust gases is adsorbed by the adsorbent, after the start of the engine, in accordance with the humidity at the start and the like, with reference to a normal adsorbent which has not been deteriorated, and measures a time actually taken by the detected humidity to rise by the predetermined value. Then, when the measured time is shorter than the predetermined time, the adsorbent is determined to be deteriorated from the fact that the rising speed of the detected humidity is higher or the detected humidity begins to rise at an earlier timing than when a normal adsorbent is used.
In addition, when the internal combustion engine is cold started, a rapid warm-up control may be conducted after the start mainly for purposes of activating a three-way catalyst at an earlier stage. Generally, the rapid warm-up control involves determining a retard amount for an ignition timing in accordance with the temperature of the internal combustion engine at the start, and controlling the ignition timing in a retarding direction to increase the temperature of exhaust gases.
However, the deterioration determining apparatus described above could fail to ensure a sufficient determination accuracy for the deterioration of the adsorbent, because the humidity sensor presents detected humidities at different rising speeds when the rapid warm-up control is conducted and when it is not conducted.
More specifically, for example, when an internal combustion engine is cold started, heat generated by exhaust gases is taken away by an exhaust system which has substantially the same temperature as the temperature in a starting environment (outside air temperature), so that the temperature of the exhaust gases is lower at a location more downstream of the exhaust system. Then, as the temperature decreases to the dew point (for example, 50-60xc2x0 C.), moisture in exhaust gases begins condensing and attaches on the inner surface of an exhaust pipe and the like, so that the moisture in exhaust gases decreases more at a location more downstream in the exhaust system. Such condensation occurs more, at an earlier time, and at a more upstream location in the exhaust system as the exhaust system is at a lower temperature upon starting. For this reason, the detected humidity tends to present a lower rising speed because the adsorbent is supplied with exhaust gas with less moisture, i.e., with a lower humidity when the condensation occurs at a location upstream of the adsorbent.
When the rapid warm-up control is conducted during such a cold start, exhaust gases are at higher temperatures than when it is not conducted, so that the moisture in exhaust gases begins condensing later at a more downstream location in the exhaust system. Therefore, when the condensation occurs near the adsorbent or at a location downstream of the adsorbent, the adsorbent is supplied with exhaust gases, the humidity of which is not reduced, unlike when the rapid warm-up control is not conducted, so that the detected humidity tends to present a higher rising speed, and this tendency is intensified as the ignition timing is further retarded.
While the detected humidity presents a rising speed which varies depending on whether or not the rapid warm-up control is conducted, and depending on the retard amount for the ignition timing when the rapid warm-up control is conducted, the aforementioned deterioration determining apparatus merely sets a time elapsed from the start as a parameter for determining a deterioration of the adsorbent, so that it only provides an insufficient accuracy for determining the deterioration, and therefore leaves a room for improvement in this respect.
The present invention has been made to solve the problems as mentioned above, and it is an object of the invention is to provide a state determination apparatus for an exhaust gas purifier which is capable of accurately determining the state of the exhaust gas purifier, which includes an adsorbent for adsorbing hydrocarbons, including a deterioration of the adsorbent, in accordance with an ignition timing of an internal combustion engine.
To achieve the above object, the present invention provides a state determining apparatus for an exhaust gas purifier arranged in an exhaust system of an internal combustion engine for determining a state of the exhaust gas purifier including an adsorbent capable of adsorbing hydrocarbons and moisture in exhaust gases. The state determining apparatus is characterized by comprising a humidity sensor arranged at a location downstream of the adsorbent in the exhaust system for detecting a humidity of exhaust gases; and adsorbent state determining means for determining a state of the adsorbent in accordance with the humidity of exhaust gases detected by the humidity sensor and an ignition time of the internal combustion engine.
According to the foregoing configuration, the humidity sensor detects the humidity of exhaust gases at a location downstream of the adsorbent of the exhaust gas purifier, and the adsorbent state determining means determines the state of the adsorbent in accordance with the result of detection, and an ignition time of the internal combustion engine. Since the abilities of the adsorbent to adsorb hydrocarbons and moisture is in a proportional relationship, the humidity detected by he humidity sensor has a high correlation to actually adsorbed hydrocarbons in the adsorbent. Also, as mentioned above, since exhaust gases vary in temperature depending on whether or not the rapid warm-up control is conducted depending on the ignition time, and on a retard amount of the ignition time when it is conducted, the humidity detected by the humidity sensor rises at a varying rate. Therefore, when the ignition time is used as a parameter in addition to the humidity of exhaust gases passing through the adsorbent for making the state determination, the state of the adsorbent can be accurately determined, including adsorption and desorption of hydrocarbons in the adsorbent, deterioration of the adsorbent, and the like, while the actual temperature of exhaust gases is reflected to the state determination.
Preferably, the foregoing state determining apparatus for an exhaust gas purifier further comprises calory calculating means for calculating calory supplied from the internal combustion engine to the exhaust system after the internal combustion engine is started, wherein the adsorbent state determining means further determines the state of the adsorbent in accordance with the calory calculated by the calory calculating means.
According to this preferred embodiment of the state determining apparatus, the calory calculating means calculates the calory supplied by the internal combustion engine to the exhaust gas after the start for determining the state of the adsorbent additionally in accordance with the calculated calory. It is therefore possible to further accurately determine the state of the adsorbent, while a varying (rising) temperature in the exhaust system and adsorbent after the start of the internal combustion engine is additionally reflected to the state determination.
Preferably, in the foregoing state determining apparatus for an exhaust gas purifier, the calory calculating means calculates the calory in accordance with the amount of fuel supplied to the internal combustion engine after the start and the ignition time.
According to this preferred embodiment of the state determining apparatus, the calory supplied to the exhaust system can be appropriately calculated in accordance with the amount of fuel supplied to the internal combustion engine after it is started, and the ignition time. Since the amount of fuel and the ignition time are known as parameters for controlling a combustion engine, the calory supplied to the exhaust system can be readily calculated using these control parameters without the need for additional sensors.
Preferably, the foregoing state determining apparatus for an exhaust gas purifier further comprises temperature state detecting means for determining a temperature state in the exhaust system; and threshold determining means for determining a threshold based on the temperature state in the exhaust system detected at the time the internal combustion engine is started, wherein the adsorbent state determining means determines the state of the adsorbent based on a result of comparison between the calory and the threshold when a change in the value detected by the humidity sensor after the internal combustion engine is started exceeds a predetermined value set therefor.
According to this preferred embodiment of the state determining apparatus, the state of the adsorbent can be determined when a change in the value detected by the humidity sensor after the internal combustion engine is started exceeds the predetermined value, i.e., at an appropriate timing at which the humidity of exhaust gases downstream of the adsorbent has sufficiently increased (risen) as the adsorption to the adsorbent proceeds to saturation. Also, the state of the adsorbent is determined based on the result of a comparison of the calory supplied to the exhaust system from the start to that time with the threshold determined by the threshold determining means. Since this threshold reflects the temperature state in the exhaust system at the start of the internal combustion engine, it is possible to more accurately determine the state of the adsorbent based on the result of comparison between the calory supplied to the exhaust system and the threshold, while actual temperature states of the exhaust system and adsorbent at the time the internal combustion engine is started, and after the start are reflected to the state determination.
Preferably, in the state determining apparatus for an exhaust gas purifier, the temperature state in the exhaust system is a temperature of cooling water when the internal combustion engine is started.
According to this preferred embodiment of the state determining apparatus, the temperature of the cooling water for the internal combustion engine when it is started can be used as a good parameter indicative of the temperature state in the exhaust system. Generally, since an internal combustion engine is provided with a water temperature sensor for detecting the temperature of a cooling water in order to detect an operating state of the internal combustion engine, such an existing water temperature sensor can be used to implement the temperature state detecting means at a low cost.
Preferably, the state determining apparatus for an exhaust gas purifier further comprises ambient temperature detecting means for detecting an ambient temperature of the humidity sensor; and relative humidity calculating means for calculating a relative humidity of exhaust gases from an output of the humidity sensor in accordance with the detected ambient temperature.
According to this preferred embodiment of the state determining apparatus, since the humidity of exhaust gases is calculated from the output of the humidity sensor in accordance with the ambient temperature around the humidity sensor, the relative humidity can be appropriately found as compensated for the temperature. Consequently, the state of adsorbent can be appropriately determined in accordance with the relative humidity found in this manner.
Preferably, in the state determining apparatus for an exhaust gas purifier, the adsorbent state determining means includes adsorbent deterioration determining means for determining a deterioration of the adsorbent as the state of the adsorbent.
As described above, a deteriorated adsorbent suffers from lower abilities to adsorb moisture as well as hydrocarbons, so that the humidity of exhaust gases rises earlier at a location downstream of the adsorbent. Therefore, according to the preferred embodiment of the state determining apparatus, a deterioration of the adsorbent can be accurately determined by the state determining technique of the present invention so far described, while the temperature states in the exhaust system and exhaust gases at and after the start of the internal combustion engine are deeply reflected to the state determination.
Preferably, in the state determining apparatus for an exhaust gas purifier, the adsorbent comprises zeolite.
According to this preferred embodiment of the state determining apparatus, the zeolite adsorbs moisture as well as hydrocarbons, and there is a high correlation between the abilities of the zeolite to adsorb both components, so that the advantages and effects so far described can be well provided by applying the present invention. The zeolite can implement an adsorbent which excels in heat resistance and is less susceptible to deterioration, as compared with, for example, silica gel, active carbons or the like when they are used as the adsorbent.